Hopefully by now you've already gotten some help?
Unfortunately, the day you stopped taking the medicine & your sore throat was returning was the day you would have wanted to call the doctor who prescribed the initial treatment and reported it. When an antibiotic isn't strong enough or isn't taken for long enough for the infection, this can set up potential for a super-infection which takes something stronger to knock it out with the next attempt.
I hope the doctor called you with the results of your throat culture. If she failed to do this, calling her office, asking for those results and now, in addition explaining the situation would be in order. The point of the culture is to see what kind of bacteria (if any) she's dealing with so that she might see to it that the appropriate and best antibiotic she can prescribe is prescribed for you. Evidently, what she prescribed was not strong enough to knock out your problem, since you completed the whole thing.
You might find her willing to prescribe something more if you call and explain the situation without requiring you to be seen again, if she has pertinent throat culture results that says what it is she's dealing with.